RepRecs are a proposed whole new class of devices that are meant to bring automated pick and place assembly to the masses.
They could complement the existing class of RepRaps with one more (the second) assembly
level (getting a hierarchy of macroscale convergent assembly started).

The proposed class of RepRecs:

are different from any currently (2017-02) existing pick and place robots by several peculiar traits (traits outlined further down).

may break a vicious cycle that attracts people away from RepRap self replication (more on that further down).

Full self replication has two parts:

automated making (printing) and

automated assembling

... the latter is still missing.
While a RepRap (per definition) can 3D-print its own parts for a copy of itself a RepRec (per definition) can assemble its 3D-printed parts to a copy of itself to a RepRap (or to an upgraded version of itself).

Symbiosis between RepRaps and RepRecs:
As there is a symbiosis between RepRaps and humans (the idea promoted by Adrian Bowyer)
there can be a symbiosis between RepRaps and RepRecs:

RepRaps make parts for RepRecs (most of them)

RepRecs assemble RepRaps (most of them)

So things that should be assemblable with RepRecs include (among many other products)
suitably/appropriately designed RepRaps (that is ReChain based RepRaps - more on that later)
These special RepRaps then in turn can pre-print the various base parts for RepRecs.

While pre printed parts are "vitamins" for RepRecs they are not "vitamins" for RepRaps.
Also there may be more efficient methods for base part preproduction like e.g. resin casting (done by RepRecs).
So one might refer to these base parts as "quasi vitamins"

Defining traits of RepRecs

Many components in narrow size range

RepRecs are constituted out of a large number of monolithic components. The size and aspect ration of all the components types lies in a narrow range. This is to make automated assembly with limited means much easier. Components have an average total-size and detail-size that lies in the lower end of the optimal range of the fabrication technology. For FDM printing that means the components on average are mid size to small (like a golf ball with nipples) but not tiny (like M3 screws with almost microscopic threads) or giant (like long threaded rods or extruded aluminum profiles).

How tiny metal screws and big structural element vitamins can best be replaced medium sized printed parts is outlined on the ReChain Frame System page.

Thoroughgoing abandonment of frictionusage for selfholding

Just as ReChain Frame SystemsRepRecs thoroughgoingly refrain from the use of friction for self holding. No compromises are to be made here.
This rather extreme design choice massively reduces the likelihood of structural failure at the contacts of the components.
E.g. it prevents undetected structural loosening that creepingly degrades accuracy in operation).
Because the system is composed from a big number of components very low likelihood of failure per component is important.
Cheap FDM printing has inaccuracies which are most often irreproducible between the usage of different machines.
These make friction fittings unpredictable and unreliable. Also without a (by requirements vitamin free) torque wrench friction is pretty much not quantifiable.
As a consequence of the "no friction usage" design decision the conventional use of screws nylock-nuts wedges and stuff like that is banned.
The fastening together of components is instead done by tension locked with smartly arranged clips. No slippage - all or nothing.
More details about how this works exactly can be found on the factored out ReChain Frame System sub-concept.

Near term macroscale RepRecs mainly assemble cheap FDM 3D printed parts but that does not necessarily exclude parts produced by other means like potentially quicker mass casting.

Relation to stiff nanorobotics

At the nanoscale there is near zero friction and violent thermal rattling potentially knocking everything loose.
So there friction can't be used by nature and thus this method is the only way to go for far term advanced gemstone based nanosystems.
The alternatives are either:

relying on relatively weak interaction forces (VdW) which should be at least feasible for "low" performance products or

making giant monolithic covalent systems - which is a horrible recycling disaster when elements like nonburnable silicon become involved.

Other less important aspects

Design decision: stationary motors

Mounting the motors mobile onto moving axes (serial mechanics) may save some complexity as can be seen on the dollo 3D-printer but:

(The ulterior motive) At the nanoscale predicting mechanical behavior (quite classical) is much easier than predicting electrical behavior (quite quantum mechanical). Thus by factoring out electrical parts of the self replicating robot the design is more likely to be scaleable down to the nanoscale.

Reducing human effort in the self replication cycle

The problem: A vicious cycle!

Manual assembly labor forces the part-count of RepRaps to be low and consequently parts to be in a wide range of sizes.

Parts in a wide size range forces manual assembly.

The solution: Breaking the vicious cycle making a virtuous cycle.

Automated assembly allows to use very many small parts in a narrow size range. (No more minimization of part count.)

Part count - size range - relationship

Fewer parts means smaller parts because small screws are the standard way we assemble things (there is probably some deeper reason here ...).

Reconfigurability boon

With more parts one gains the great benefit of very quick and cheap drastic reconfigurability in geometry.
A simple change of linkage arm-length may be a matter of minutes instead of hours.
Cartesian-robots, delta-robots, scara-robots and whatnot may share many of the same parts!
No reprinting and throw-away. Just reassembly and reuse.

Fewer vitamins - slight in number - big in count ratio

RepRec pick-and-place-robots have less vitamins than RepRaps.
They have no hot-end, no filament-drive and no print-bed so more of their parts can be printed.
(Well for the motors an attempt could be made to try a pneumatic actuator solution with printed TPU bellows but this is probably to slow and weak for productive usage.)
But RepRaps are equally important in the the full self replication cycle. At least if they are not replaced by RepRec operated part casting or a pre-produced stock of parts (that maybe still assembled in some old structure).

RepRaps produce parts waay slower than RepRaps can consume them.

If the RepRap to RepRec ratio is choosen to avoid a bottleneck (this would be RepRap a botfarm - very unlikely in DIY home settings) there is little potential in saving vitamins.

If one chooses one RepRap and one RepRec halve of the printer specific vitamins are saved.

If the same machine is used both as RepRap and RepRec the needed vitamin-count obviously unchanged from a "normal" (screw-free, frame-printed, RepRec-compatible) RepRap.

In terms of part-count (not in terms of volume or mass) the vitamin to printed-part ratio can probably go way below 1%.

The parts

A good size range for the parts of a RepRec would be ~5mm ... ~5cm (factor 10) compared to
0.3mm(M3-thread) ... 300m(length of threaded rod) (factor 1000) in many 3D printers.

Possible Designs

Design suggestion: parallel SCARA robot + 3+1-DOF rotation head

The FANUC M-1iA/0.5A robot has an interesting head serial mechanics 3-DOF rotating head that can turn all three hinges the full 360°.
The internal workings of the head are very fine-grained and will by far not meet the "avoidance of small or special material parts" requirement for the RepRec.
With major redesign it should be possible to meet this requirement. The head will become quite big and heavy though (wild guess: a little less that 20cm in diameter)

Beside the nice head there are problems with the delta-bot design though.

There are a lot of non-planar hinges which are difficult to make compact with the design restriction of small component avoidance.

A self replicating robot needs a quite big working volume to build its copies. To make it bigger the FANUC rotative arm drives in the top cupola could be changed to a linear rail design. But then coupling the three rods that drive the rotative head with statically mounted drive motors at the top will become very space wasting (long overstanding torsion-drive-rods). Thin torsion rods are a bad choice for low stiffness plastic anyway.

Parallel SCARA instead:
What kind of robot design would lend itself well if one wants to transmit all the mechanical motions exclusively via chains to the top entrance side of the FANUC-style serial mechanics rotating head?

One possibility is a parallel SCARA design (similar to RepRap Morgan). Unlike a delta robot only hinges in two dimensions are present. It is easy to thread a chain drive through such a 2D-hinge by double-sprockets (to further hinges or like here to the head). Actually arbitrary many chains can just by stacked with just one design. Also with a parallel SCARA robot there's no need for chains to do two-dimensional translations via statically mounted motors. The parallel SCARA arms can be driven directly.

It is natural to supply the rotative head with a fourth chain (two chains of each side of the parallel SCARA bot).
The one additional rotation available at the tip of the serial mechanics rotative head can be used for both clamping action and screwing action.

Adding a fifth or even a sixth chain should probably avoided since the head will get way to big, heavy, hard to assemble, unstable and whatnot. (Each DOF adds an onion-like shell to the head). Instead statically mounted simplistic counter actuators that are reachable from within the work-space (a simple possibly mechanically multiplexed clamp-board) should help a lot.

The hinges of a SCARA and the thereon stacked gear-bearings for chain sprockets can be made from split-sun-post-assembly-gear-bearings. By tensioning the whole bearing stack (clamping all the suns wit a common axle) the gap-backlashes of the bearings can be fully reduced to zero.

Compared to a delta-bot a SCARA design may have some minor shortcomings like:

lower stiffness

the scissor mechanism may be more in the way that a delta platform

the z-axis must be handled separately (could be a good thing)

The double tripod

A design similar to Ralph Merkles double-tripod - just with linear drives
It can be found here: Zyvex nanotech 6dof there's a vrml 3D model available
This is a compromise between:

the steward plattform (high stiffness low range of motion) and

the serial robotic arm (low stiffness high range of motion)

FANUC style robot

A design similar to the "FANUC M-1iA/0.5A" robot - just with linear drives.
That is a chain driven linear rail delta-bot for bigger working area (instead of rotative direct stepper drive)

Maybe with the differences of a chain driven rotating head instead of the torsion rods??

Comparison of goals with other Robots

This is not a judgement of the projects - they have different goals.
This is a judgement about how suitable design decisions in those projects might be for RepRec projects.

The BrickRap is mostly composed of lots of small Bitbeam parts. A working RepRec could be made capable to assemble such LEGO like structures but it would do so only as a secondary product. A RepRec in contrast to the BrickRap does not depend on friction fittings to lock its own parts together. A RepRec also tries to concentrate the clip lockings (energy barriers) and and minimizes their number.

Distinction to other self replicating pick and place robots

There is Matt Moses et al. prefabricated block based "self replication modular manufacturing system".
Here is a paper about that system: "An architecture for universal construction via modular robotic components" (UCVMRC)
While this is probably currently (2018) one of the projects that comes closest to the proposed class of RepRecs
it is rather far away from what would count as a RepRec, maybe going into a different direction.

There's an extended mother Tron-Recognizer-like-3DOF-assembler-tower on two rail-tracks.

It is extending its own rail-tracks. It assembles rail-tracks normal to the ones that it uses itself.

It builds up an non-extended child Tron-Recognizer-like-3DOF-assembler-tower onto the new tracks that run normal to the one itself runs on.

A little earlier in the video there's a bit of introduction noting the concept of "parts complexity" (not necessarily related to the somewhat intuitive and unfortunately less formal use of "complexity" used here in the following text).

Main differences and similarities between UCVMRC and the proposed class of RepRecs:

UCVMRC uses screws for assembly that are held in place due to friction forces (like pretty much every design in existence today 2018). A good RepRec would not do this but would be based on a friction usage abolishing ReChain Frame System instead. (The reasons are elaborated in preceding text)

While UCVMRC already aims at more heterogeneous part diversity than many other approaches RepRecs still would have much more diversity of part types (more external exposed complexity per part) more on that later in the adapter section.

While UCVRMC already aims at not including too much functionality in the base parts RepRecs still would have simpler smaller more passive parts (less internal enclosed complexity per part) more on that later in the adapter section. In particular RepRecs base parts should manage without any electrical components. (No wiper contacts, no flexing wires compensating relative motions).

UCVMRC and structures built by it are rather anisotropic since parts all face in the same main axis (upwards). RepRecs should not have that limitation. Means to change direction are a requirement.

UCVMRC suffers a bit from geometry that is far from ideal for maximum structural stiffness (moving masses on "levers" that stand off 90°). RepRecs should be designs such that such situations are avoided.

Just as UCVMRC Reprecs may operate on a lattice that they can extend by themselves. But instead of a 2D-grid RepRecs would aim at stiff 3D-trussworks (octett truss). So instead of track-extension there's truss-extension (in all three spacial dimensions).

Both UCVMRC and Reprecs operate in precisely defined structured environment (low entropy, isentropic, machine phase) they can operate without feedback (without "looking" at what they are doing) in open loop control.

For locomotion of the main manipulating part of a RepRec system in a the Truss part of a Reprec System it mey be desirable to try to avoid linear rail designs and go for rotative brachiating locomotion instead (this is not absolute requirement).

While there is most certainly the intention to design and build a build a demonstartion instance of a RepRec system, the RepRec concept in its full generality is not a project with a strict deadline for such a demonstration instance. This likely excludes most means for funding but on the other hand makes an approach possible that does not allow any project breaking compromises to creep in.

Gripping adapters - moving internal to external complexity

Self replicating pick-and-place robots with very low component/part specialization (that is very view generic part types like in Matt Moses design) would be useful for practical use if the constituent parts are small enough such that the created structures can be treated as a mechanical metamaterials. In this context small means almost imperceptible small by human senses. That is the parts must be at least under the 300 micron scale.

The purpose Matt Moses macroscopic design:

Perfectly demonstrates the principle possibility of high degree self replication in a system of manageable complexity.

It is impractical for making almost all of everyday utility stuff. At the macro-scale a robot that is using too few types of components/parts (including non included but handleable parts) just degrades to a "nonproductive replicator". A device that only exists for the sake of demonstrating the possibility of self replication. Without producing "nectar" a robot design has no chance of explosive growth.

To get a useful productive self replicating macro-scale pick-and-place robot there are two major things:

(1) More external complexity (exposed for external function in post assembly time) must be invested in part prefabrication.

That is compromises on component/part design must be kept minimal.
This is easy with 3D-printing where the "voxels" are under the required 300 micron for cube like blocks with limited external functionality.

Still the compromise of trying to reuse part-types (the same part design) is necessary to keep the number of part types and associated adapters low (wild guess: <300 ?)

(2) Less internal complexity (enclosed for internal function in assembly time) must be kept in the design of the parts.

That is the parts can't be made to all have a point where they can be picked up with the same manipulator.
Encoding The gripper-shape in all the parts is massive physical code duplication and leads to excessive physical space overhead.
The interfaces to standard gripper(s) must be separated from the parts - especially the smaller ones.

The result is the need for a sufficiently wide set of part adapters.
Or looked at it from the perspective of the manipulator head a set of specialized end effectors coupleable to the generic manipulation head(s).

Adapters of adapters may come up occasionally. Further stacking probably not.
Obviously the adapters need to be well tensioned to the manipulation head when used.

Special parts / adapters / assemblies

part adapters

part magazines

temporary clipping tools (to not need multiple manipulators at the same time - only for use in "assembly time" removed before "use time")

tensioning tools (akin to screwdriver bits -- Note again: No locking of screws by friction intended in a RepRec!)

DOF restrictors for not yet mounted but already assembled chains (e.g. a LIFO chain coil magazine from which chains can be rolled onto sprockets while permanently keeping tension! This is difficult but important. End-effector part assemblies for locking chains to loops are their own topic.)